1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and separating fine solids from vapor streams.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Most FCC's units are all riser cracking units. This is more selective than dense bed cracking. Refiners maximize riser cracking benefits by going to shorter residence times, and higher temperatures. The higher temperatures cause some thermal cracking, which if allowed to continue would eventually convert all the feed to coke and dry gas. Shorter reactor residence times in theory would reduce thermal cracking, but the higher temperatures associated with modern units created the conditions needed to crack thermally the feed. We believed that refiners, in maximizing catalytic conversion of feed and minimizing thermal cracking of feed, resorted to conditions which achieved the desired results in the reactor, but caused other problems which could lead to unplanned shutdowns.
Modern FCC units must run at high throughput, and run for years between shutdowns, to be profitable. Much of the output of the FCC is needed in downstream processing units. Most of a refineries gasoline pool is usually derived directly from the FCC unit. It is important that the unit operate reliably for years, and be able to accommodate a variety of feeds, including very heavy feeds. The unit must operate without exceeding local limits on pollutants or particulates. The catalyst is somewhat expensive, and most units have several hundred tons of catalyst in inventory. Most FCC units circulate tons of catalyst per minute, the large circulation being necessary because the feed rates are large and for every ton of oil cracked roughly 5 tons of catalyst are needed.
These large amounts of catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst and fines. Catalyst and fines must also be removed from flue gas discharged from the regenerator. Any catalyst not recovered by the regenerator cyclones stays with the flue gas, unless an electrostatic precipitator, bag house, or some sort of removal stage is added at considerable cost. The amount of fines in most FCC flue gas streams exiting the regenerator is enough to cause severe erosion of turbine blades if a power recovery system is installed to try to recover some of the energy in the regenerator flue gas stream.
The solids remaining at this point are exceedingly difficult to recover, having successfully avoided capture despite having passed through several stages of highly efficient cyclones. The solids are very small, essentially all of the solids are below 20 microns, and including significant amounts of sub-micron and micron to under 5 micron material.
Recovery of such solids has been a challenge for almost a century. A survey of the state of the art is described in Perry's Chemical Engineering Handbook, in DUST-COLLECTION EQUIPMENT, abstracted hereafter. A gravity settling chamber could be used, but generally only works for particles larger than about 40 microns in diameter. Small particles have a long settling time and are swept out before they settle, unless the device has a large cross-sectional area.
The Howard dust chamber improved things a bit by providing multiple horizontal plates in the chamber, so that the dust did not have so far to fall. This arrangement is shown in U.S. Pat. No. 896,111, 1908. For an FCC regenerator, with large volumes of regenerator air, and large amounts of fines and dust, a settling chamber with a larger footprint than the entire FCC unit including main fractionator would be required.
Impingement separators improved things a bit, by using inertial forces to drive particles to impinge on collecting bodies in the gas stream. These work well for particles above 20 microns, but have little effect on the dust in FCC regenerator flue gas.
Cyclone separators are settling chambers in which gravitational acceleration is replaced by centrifugal acceleration. FCC regenerators use large cyclone separators, and are able to efficiently recover essentially particles larger than about 5 microns. Collection efficiency is poor for smaller than 5 micron sized particles, and very poor for anything smaller than 2 or 3 microns. To increase collection efficiency in FCC regenerator cyclones, refiners have accepted higher pressure drops and forced incoming gas to make 4 or 5 turns in the cyclone.
Refiners with large FCC units typically use 6-8 primary and 6-8 secondary cyclones in their FCC regenerators, because of mechanical constraints and pressure drop concerns. These inherently let a large amount of fines and dust, in the submicron to 2-3 micron size range, pass out with the flue gas. This material must be removed from the flue gas prior to discharge to the atmosphere, or passage through a power recovery turbine.
Generally a third stage separator is installed upstream of the turbine to reduce the catalyst. loading and protect the turbine blades, or permit discharge of flue gas to the air. When a third stage separator is used a fourth stage separator is typically used to process the underflow from the third stage separator. The fourth stage separator is generally a bag house.
Third stage separators typically have 50 or 100 or more small diameter cyclones. One type of third stage separator is described in "Improved hot-gas expanders for cat cracker flue gas" Hydrocarbon Processing, March 1976. The device is fairly large, a 26 foot diameter vessel. Catalyst laden flue gas passes through many swirl tubes. Catalyst is thrown against the tube wall by centrifugal force. Clean gas is withdrawn up via a central gas outlet tube while solids are discharged through two blowdown slots in the base of an outer tube. The device was required to remove most of the 10 micron and larger particles. The unit processed about 550,000 lbs. hour of flue gas containing 300 lbs hour of catalyst particles ranging from sub-micron to 60 micron sized particles.
Third stage separators are also shown in U.S. Pat. Nos. 4,285,706 and 4,755,282 which are incorporated by reference.
Third stage separators typically use large numbers of horizontally mounted small cyclones. This device is downstream of and external to the FCC regenerator. Several vendors (Polutrol and Emtrol) supply systems with many small diameter, horizontally mounted, closely connected and radially distributed cyclones about a central gas outlet.
Although third stage separators help, they have never been as efficient as desired, and do not respond well to large catalyst or pressure surges from the FCC regenerator. They also leave too much fines in the flue gas, and discharge too much gas with collected fines. Many refiners have had to install electrostatic precipitators or a baghouse on underflow downstream of the third stage separator to reduce fines emissions.
Conventional third stage separators, based on multiple cyclones, have other problems as well, one of the more significant being poor response to a catalyst load dump. If something goes seriously wrong in the FCC unit, e.g., a flow reversal when hydrocarbon feed enters the regenerator followed by massive amounts of steam quench, then all the catalyst in the regenerator can be lost. Thus if some event causes high catalyst losses, or low catalyst levels in the regenerator which expose the cyclone diplegs, the cyclones are effectively turned off, and all the catalyst is blown out. Essentially the entire catalyst inventory can be blown up the stack and settle over a period of hours or even days. When a power recovery turbine is used the turbine blades will be severely eroded by passage of massive amounts of catalyst through the turbines.
U.S. Pat. No. 4,392,345, incorporated by reference, discloses a way to control bypassing of flue gas around turbines when a catalyst load dump occurs.
We wanted a better third stage separator. We wanted to achieve a breakthrough in fines recovery, and come up with a robust design that could tolerate large catalyst or pressure surges without damaging turbines or distributing tons of catalyst across the countryside.
We believed that third stage separators based on cyclones were at their limit. The laws of physics made collection of fines and dust difficult. Going to multiple, small diameter cyclones helped, but was by no means a complete solution to the problem. They also responded poorly to catalyst surges, which are recurrent events.
Local environmental regulations generally prohibit intermittent, as well as continuous, discharge of particulates into the atmosphere. Power recovery turbine blades are degraded slowly by modest amounts of fines, and quickly by surges. We thus wanted a higher efficiency of fines recovery, and a way to tolerate sudden surges of fines and/or catalyst.
Electrostatic precipitators were a possibility, but even these were not so effective on micron and submicron particles. Precipitators are also large, expensive and require periodic shutdowns for maintenance.
We considered filtration. Filter elements, preferably sintered metal or ceramic, but possible including fabric filters, seemed to present the best possibility for a reliable, low maintenance, surge resistant system. The existing designs based on filtration had problems. Some refineries now use filters as a fourth stage separator feeding underflow from the 3rd stage separator, typically downstream of a third stage separator using small diameter cyclones.
In these fourth stage separators the flue gas discharges up into the center of a large vessel containing many filter elements. The filters are exposed to upsets in catalyst loading. If the filters are coated, or overloaded, with a thick layer of fines, the pressure drop across the filter elements can build rapidly, leading to a pressure surge which cascades upstream to the FCC regenerator or reactor. This can reduce FCC throughput or even shut down the regenerator air blower, which also shuts down the FCC unit.
We wanted to retain many elements of this design, but modify the way fines laden gas was added to improve the efficiency and reliability of the operation. We also wanted to eliminate most of the particulates emissions currently associated with FCC units. We believed we could even remove more than 90%, and up to 99% of the particulates emissions commonly generated by FCC units. Not only would this represent a substantial reduction in particulates emissions, it would also greatly improve the reliability of power recovery turbines, if present.
We discovered a way to use filtration to greatly improve third stage separation, and perhaps eliminate the need for a fourth stage separator. We discovered that adding gas via an annular inlet allowed a significant amount of inertial separation to occur upstream of the filter elements. It also allowed us to impose a generally downward flow of gas and fines as they flowed to the filter elements. An additional benefit is that radial out-to-in flow exposes the maximum surface area of filter elements to incoming gas, in contrast to the old approach with exposed the minimum amount of surface area. These relatively simple, and easy to implement, changes greatly improved the efficiency and reliability of the device.
We achieved a low grade of particulates removal via an initial inertial separation, and a measure of surge protection. We then used both downflow settling, and filtration, together, to remove an extraordinary amount of fines from a gas stream.